Integrated circuit components are used in various kinds of electronic devices used in day-to-day life such as computers, electronic appliances, television, mobile phones, cars, and so on. These components include without limitation microprocessors, microcontrollers, digital logic circuits, A/D and D/A converters, memory devices, display devices etc. The processing within the integrated circuit components as well as interaction between the various integrated circuit components in a circuit needs to occur in synchronization with predetermined time reference. This time reference is often called as “clock”. Typically, the clock is a sequence of electric pulses which occur at regular interval (e.g., every 1 microsecond for 1 MHz clock, every 1 nanosecond for 1 GHz clock etc.). These pulses are generated by clock generator and supplied to integrated circuit components in the circuit to provide them with common time reference.
The clock generator often includes a circuit component called Phase Locked Loop (PLL). The PLL generates an output signal of a frequency which is locked to (multiple of) the input signal frequency. The input signal typically comes from a stable frequency reference such as “quartz crystal”. The PLL detects any difference (error) between the output frequency and the input frequency, and changes the output frequency to eliminate the error. This is typically achieved by generating a control signal which is related to the error and feeding this control signal to Voltage Controlled Oscillator (VCO). VCO is a component whose output frequency is controllable (changeable) by applying proper control signal (control voltage).
Though VCO has been described in clock application, it also has several other applications such as reference frequency generator in radio circuits etc. In general, the VCO needs to have certain desirable properties such as described below, in addition to others:
Low jitter: The clock output by the VCO must have substantially equispaced zero-crossings. There should be minimum variation in the time duration between two successive zero-crossings over time.
Low conversion gain constant (KVCO): For low jitter applications, the conversion gain constant (KVCO) which is the ratio of the change in the VCO frequency to the change in input voltage should be small. This ensures that noise created by stages preceding the VCO does not unduly affect the overall clock jitter.
Sufficient tuning range: The tuning range of the VCO (the minimum to maximum frequency it can reach in all conditions) should be large enough to accommodate the ranges desired for applications.
Minimal variation of the conversion gain constant (KVCO) with variation in process corner, supply voltage, and temperature: This is necessary to maintain the stability of the PLL as the KVCO is a critical parameter contributing to the overall loop bandwidth.
Clearly, techniques for implementing VCO which can exhibit one or more of the properties described above and other properties are highly desirable.